1. Field of the Invention.
The present invention relates to corrugated tubing generally and, more specifically, to a novel method and apparatus for making the same, the corrugated tubing produced by which offering a range of both expansion and contraction not available with conventionally manufactured corrugated tubing.
2. Background Art.
Corrugated metal tubing has utility in such varied applications as pressure and thermal sensors, seals, expansion joints and chambers, and vibration dampeners. The convolutions of the corrugated tubing are typically formed by mechanical means from welded thin-wall tubing after the tubing is formed. Such corrugated tubing is manufactured, for example, by Westport Development Manufacturing Company, Orange, Conn.
Known methods of forming corrugated tubing are of two types: hydroforming and welding. Hydroforming itself includes three methods. In one method, a thin-wall tube sealed at one end is inserted into an apparatus which includes a plurality of spaced apart annular disks, each formed of two separable sections, the disks being spaced apart a relatively large distance. Pressure is applied to the open end of the tube, thus causing the wall of the tube to bulge into the spaces between the disks. The disks are then drawn toward each other to form the corrugations and then the sections of the disks are removed. This method is relatively quick and inexpensive, but the corrugated tubing thus produced is not very uniform. A second method is a variation of the first, in which, rather than pressurizing the tube a rubber cylinder is inserted into the tube and the rubber cylinder is then compressed, thus forming the bulges between the annular disks. The latter method is typically used for very large diameter corrugated tubing. In the third method of hydroforming, the convolutions are formed one at a time by hydraulically forming a bulge between a chuck plate and a shuttle. The shuttle is then moved toward the chuck plate to form a convolution, or corrugation, having a desired crest radius at its periphery and a desired trough radius between it and an adjacent convolution. This process is repeated along the tube until the desired number of corrugations is formed.
The third method of hydroforming is described in U.S. Pat. No. 3,141,496, issued Jul. 21, 1964, to Yowell et al, titled APPARATUS AND PROCESS FOR FORMING CORRUGATIONS IN TUBING, the disclosure of which and the references cited therein are hereby made a part hereof by reference.
The welded plate method comprises forming a number of thin metallic annular disks. The disks are then put in forming dies which bend the disks so that, when the disks are stacked, alternating pairs of disks meet at either their inner or their outer peripheries. The contacting inner and outer peripheries are then welded, while using copper chill rings to prevent distortion. The disks are usually provided not flat, but with a wave-shaped cross-section, frequently a sine wave, which stretches and relieves stresses as the corrugations are flexed.
A disadvantage of the hydroformed corrugated tubing is that it cannot be compressed "flat," that is, so that the corrugated tubing is only as long as the total of the individual thicknesses of metal, without destroying the spring of the bellows. This is because at each turn of a convolution, there is an internal radius of about 10 times, or greater, the metal thickness. Hydroformed corrugated tubing, however, can be relatively easily extended from the normal position and can be used in either an expansion or compression mode.
An advantage of the welded plate method is that the plates can be compressed flat because the individual segments touch and there is no internal radius where the edges of the individual segments meet. However, the welded plate method is very costly in that it requires a high amount of labor. A further disadvantage of welded plates is that they have a very low spring rate and can only be extended from their rest position a short distance and that only with a large amount of force; consequently, it is usually used only in the compression mode. The overall extension/compression ratio of welded plates is typically on the order of about 4/1, and the size of that ratio is due primarily to compression distance. "Extension/compression ratio" as used herein means the ratio of the length of the corrugated tubing extended to its maximum extent to the length of the corrugated tubing compressed to its maximum extent.
It will be understood from the foregoing that it would be desirable to be able to use relatively inexpensive hydroforming techniques to produce corrugated tubing that has the compressibility of a welded corrugated tubing, yet does not suffer from the poor extension characteristics of welded corrugated tubing. It is not possible to simply use the hydroforming techniques described to form corrugated tubing with radii having small radii, as such would result in uncontrolled radii formation, the radii would become too sharp, radial wrinkles would form on the convolutions, and there could be crushing of the convolutions. The result of the latter would be that the corrugated tubing could not be extended from its crushed position.
Accordingly, it is a principal object of the present invention to provide hydroformed, thin-wall metallic corrugated tubing which can be compressed to a length almost equal to the total of the thicknesses of the metal of which it is formed.
Another object of the invention is to provide such corrugated tubing which can be extended from its normal position with relatively small force.
A further object of the invention is to provide such corrugated tubing which has an extension/compression ratio of 4/1 and greater.
An additional object of the invention is to provide an apparatus for making such corrugated tubing.
Yet another object of the invention is to provide a method for making such corrugated tubing.
Yet a further object of the invention is to provide such method and apparatus which are economical.
Other objects of the present invention, as well as particular features and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures.